Magnesium diimidosulfinates

Although the sulfurdiimides react readily with lithium organic compounds the yield and the purification by crystallization is often difficult or time consuming. While the reaction with MeLi proceeds without problems and the methyldiimidosulfinate is obtained in good yields the reaction with EtLi and BzLi is not that straightforward.

Though the ethyldiimidosulfinate as well as the benzyldiimidosulfinate are formed (see Figure 2-25) the yields are poor and the crystallization time is measured in weeks rather than days.^[145]

Figure 2-25: Molecular structure of [(Et2O)0.5Li{(N^tBu)(SiMe3)SEt]2 (15) (left) and [(THF)0.5Li{(N^tBu)2SBz]2 (16) (right). All hydrogen atoms have been omitted for clarity.

[(Et2O)0.5Li{(N^tBu)(SiMe3)SEt]2 (15) and [(THF)0.5Li{(N^tBu)2SBz]2 (16) both crystallize as dimers with one lithium atom coordinated by the four nitrogen atoms of the ligands while the second lithium atom is coordinated by one nitrogen atom of each ligand and a donor base. Although the first ethyldiimidosulfinate synthesized by Wrackmeyer^[177]

exhibited a step-shaped structural motif without any donor base the described structural motif of 15 and 16 is well known for diimidosulfinates.^[67,68,76] Most bond lengths and angles are also in line with the literature reports. Only one of the N–S–C angles in 16 is considerably more acute than the other (105.6° vs. 98.9°). This is due to an interaction of one hydrogen atom attached to the tert-butyl group with the π-system of the benzyl substituent (see Figure 2-26). Another interesting fact is the absence of any disorder in 15. Up to now, all structures containing the tert-butyl-trimethylsilyl-sulfurdiimide exhibited a disorder of the substituents at the nitrogen atoms in the solid state.

Figure 2-26: Hydrogen bond in [(THF)0.5Li{(N^tBu)2SBz]2.

Surprisingly, in 15 no such disorder can be observed. A closer look at the structure shows that the trimethylsilyl groups are attached to the nitrogen atoms coordinating only one lithium atom while the nitrogen atoms coordinating both lithium atoms bear tert-butyl groups. The reason for this is probably the steric strain since the donor base is also interacting with the second lithium atom.

Because of the mentioned difficulties during the synthesis of the lithium diimido-sulfinates, it was tried to react different sulfurdiimides with a variety of Grignard reagents. Deuerlein already succeeded in adding a phenyl Grignard and a benzyl Grignard to tert-butyl-sulfurdiimide.^[152] To further investigate this reactions more Grignard reagents and other sulfurdiimides where employed. By adding the appropriate Grignard reagent RMgX to a sulfurdiimide, compounds of the general formula [(THF)2MgX{(NR')2SR] can be obtained in both high yield and purity (see Scheme 2-7). Storage of the solutions at -24 °C yields colorless crystals suitable for X-ray structure analysis after one day.

17 18 19 20 21

Scheme 2-7: Preparation of the magnesium diimidosulfinates 17-21.

By employing the above described reaction [(THF)2MgBr{(N^tBu)2SMe] (17), [(THF)2MgCl{(N^tBu)2SⁿBu] (18) and [(THF)2MgCl{(N^tBu)(NSiMe3)SBz] (19) could be synthesized and structurally characterized. 19 was synthesized during the work on the diploma thesis^[74] but is discussed here, too, because of the structural similarities.

In the structural discussion the three compounds are also compared with [(THF)2MgCl{(N^tBu)2SPh] (20) and [(THF)2MgCl{(N^tBu)2SBz] (21) already synthesized earlier by Deuerlein. ^[152]

So far, metal diimidosulfinates where known to crystallize mostly as dimers.^[69,71]

Various structural motifs were found but the one thing they all had in common was the dimeric aggregation. Only in the presence of a polydentate donor base like TMEDA monomers could be crystallized.^[82] [(THF)MgCl{(N^tBu)2SPh]2 (20) is no exception to this rule of thumb. It crystallizes as a dimer with a central four membered Mg2Cl2 ring (see Figure 2-27).

Figure 2-27: Molecular structure of [(THF)2MgCl{(N^tBu)2SPh] (20). All hydrogen atoms have been omitted for clarity.

Except for the central Mg2X2 ring, 20 exhibits the same bond lengths and angles within their standard deviation as its published heavier congener [(THF)MgBr{(NSiMe3)2SPh]2.^[68] Although the structural motif is different, the geometry around the metal atom is very similar in these two compounds. The preferred five-fold coordination of the magnesium atom is achieved by two Mg–N contacts to the diimidosulfinate, the interactions with the two chlorine atoms and the coordination of one THF molecule. As a result of this geometry, the chlorine atoms are not positioned at the sterically favorable apex of the square-pyramidal environment of the magnesium atoms, but reside in the base. As already observed for other compounds containing a central Mg2X2 ring the Mg–Cl interactions show a distinct asymmetry (Δ(Mg–Cl) = 0.07 Å).^[178-181] This shows that the electron density of the halogen anions is not shared equally between the magnesium atoms. The tertiary carbon atoms of the ^tBu groups are nearly in plane with the N–S–N plane which is tilted 37° towards the central Mg2X2ring. In addition both N–S–N planes are twisted 14° sidewards to avoid steric strain between the phenyl ring and the THF molecule of the other half of the dimer.

Table 2-5: Selected bond length [Å] and angles for 17-21.

Compound 17 18 19 20 21

Distances

S–N1 1.6201(59) 1.6277(12) 1.6186(15) 1.6089(17) 1.6285(13) S–N2 1.6307(54) 1.6346(11) 1.6168(15) 1.6160(17) 1.6218(12) S–C 1.8039(58) 1.8145(14) 1.8393(17) 1.8173(20) 1.8440(15) N1–Mg 2.1263(61) 2.1042(12) 2.1118(17) 2.0815(17) 2.1021(14) N2–Mg 2.0943(56) 2.1504(12) 2.1542(15) 2.0558(18) 2.1697(13)

Mg–Cl Mg-Br

2.4960(13) 2.3474 (6) 2.3407(7) 2.4160(8)

2.4841(8) 2.3430(7) Mg–O1 2.1166(47) 2.0793(11) 2.0909(14) 2.0265(15) 2.1112(12) Mg–O2 2.0736(56) 2.1116(11) 2.0866(14) 2.0854(12)

Angles

N–S–N 99.44(15)° 99.62(6)° 100.05(8)° 94.41(8)° 99.65(7)°

N–Mg–N 71.97 (0.12) 71.71 (0.04) 71.06 (0.06) 69.77 (0.07) 71.07 (0.05)

In contrast to 20, 17-19 and 21 crystallize as monomers. Although the geometry formed by five-fold coordination at the magnesium atom is maintained, the structural motif shown by 17-19 and 21 is totally different. The Mg2X2 four-membered ring is not retained but a second THF molecule completes the coordination sphere at the metal ion. The square-pyramidal environment is preserved, but in the monomeric form the halogen anion can occupy the sterically favorable apex of the pyramid.

Figure 2-28: Molecular structure of [(THF)2MgBr{(N^tBu)2SMe] (17) (left) and [(THF)2MgCl{(N^tBu)2SⁿBu] (18) (right). All hydrogen atoms have been omitted for clarity.

Different to 20, the tertiary carbon atoms of the ^tBu groups in 17-19 and 21 point away from the magnesium atom and the C–N bonds includes angles of 31° to 47°

with the N–S–N plane, providing enough space at the other side of this plane for two THF molecules and the organic substituent at the sulfur atom. In 17 (see Figure 2-28) the deviation of the ^tBu groups is much more symmetric (39.5° and 38.7°) but all other compounds, especially 18 (see Figure 2-28), exhibit a distinct asymmetry (31.2° and 47.3°) with a larger angle present at the side where the substituent at the sulfur atom is positioned.

The structures of the magnesium diimidosulfinates show that neither the nature of the halogen anions nor the organic substituents at the nitrogen atoms have an impact on the structural motif. So the reason for the formation of a monomer in favor of a dimer must be related to the organic substituent bound to the sulfur atom instead. If a sterically demanding group is present at the sulfur atom there is not enough room for this bulky group and an additional THF molecules at the same side of the N–S–N plane which would be required for a potential dimerization, even if the ^tBu groups at the nitrogen atoms are pointing to the adjacent side.

Although a phenyl group is generally not considered to be sterically particularly demanding, its bulk is sufficient enough to bring the ortho carbon atom in close proximity to the magnesium atom, leaving not enough space for both THF donor molecules. In contrast to that, the methyl group of 17 provides less sterical demand and the substituents of the other compounds all comprise CH2 spacers able to bend the remaining organic substituent away leaving enough space for the THF molecules.

Figure 2-29: Molecular structure of [(THF)2MgCl{(N^tBu)(NSiMe3)SBz] (19) (left) and [(THF)2MgCl{(N^tBu)2SBz] (21) (right). All hydrogen atoms have been omitted for clarity.

Since the THF oxygen atom is a better donor to magnesium than a halogen anion bridging two metal ions, the monomeric form is favored for 17-19 and 21. Because of the better donor the Mg–N and the Mg–O contacts are longer in 17-19 and 21 compared to 20. Only the Mg–X bond is shorter in 17-19 and 21 than in 20 because the halogen anion is not forced to share its electron density between two metal ions.

Among the monomeric compounds 17-19 and 21 the structural differences are marginal (Figure 2-30). Even the asymmetric diimidosulfinat 19 (see Figure 2-29) exhibits nearly the same bond lengths and angles as its symmetric congener 21.

Only the S–C bonds of 19 and 21 (1.84 Å) are clearly longer than that of 17, 18 and 20 or other organyl diimidosulfinates (1.80 Å on average).^[67,68,76] The phenyl substituents at the benzylic carbon atoms withdraw electron density from the S–C bonds, which are therefore destabilized and elongated.

Although 19 possesses a asymmetric sulfur atom, it crystallizes as a racemate in the centrosymmetric space group P21/c. In addition, the ^tBu groups and the SiMe3 groups are disordered making an assignment of the isomers impossible.

Figure 2-30: Molecular structure overlay of 17-19 and 21, all hydrogen atoms and the carbon atoms of the THF molecules have been omitted for clarity.

The N–S–N bond angles in all presented compounds (94.4° - 100.1°) are more acute than those in alkali metal derivatives (104.2° - 110.7°),^[68,69] but span almost the same range as in comparable compounds with magnesium atoms or other dicationic metals (94.3° - 98.5°).[68,90,139] This can be attributed to the higher charge at the magnesium dication, leading to a stronger repulsion between the positively charged sulfur atom and the metal ion.

Another interesting fact about the magnesium diimidosulfinates is their behavior in solution. The ¹H- and ¹³C-NMR-spectra all exhibit a double set of signals for 17-21.

Integration of all peaks and NOESY experiments displayed that in each double set of signals the peaks shifted to higher field belong together. Figure 2-31 shows the spectra of 17 measured at different temperatures. The signals for the methyl groups occur between 2.1 ppm and 2.3 ppm while the ^tBu protons resonate between 1.1 ppm and 1.3 ppm. The spectrum of 17 at -70 °C also displays the occurrence of an additional set of signals resulting in a triple set.

Figure 2-31: ¹H-NMR spectra of 17 at different temperatures (500.132 MHz, d8-THF).

For the measurements the temperatures as well as the elapsed time since the priming of the samples were adapted. The concentration of the samples was also varied in the different experiments. This way it could be verified that the equilibrium is time, concentration and temperature dependent. Figure 2-31 shows that with lower temperatures even more species are involved and that a higher temperature results in an increase of species a at the expense of species b. In addition, the peak of b gets sharper. When the sample is more concentrated the ratio of the integrated intensity of b divided by the integrated intensity of a is getting higher. When the elapsed time between priming the sample and the measurement increases the ratio of the integrated intensity of b divided by the integrated intensity of a is getting smaller. In addition, the peak for b is getting broader as more time passes. Since the exchange between a and b works in both directions it was verified that both species can be transformed into each other and that the double set of signals is not just due

to hydrolysis or ligand scrambling. In addition, a ¹⁵N-HMBC-spectrum was recorded which showed two cross peaks, for both ^tBu signals with the signal of the nitrogen atoms, at -270 ppm and -272 ppm respectively. The proximity of the signals indicates that the chemical environment of the nitrogen atoms is the same for both species.

This, coupled with the absence of any amine proton proves that no hydrolysis is responsible for the signal doubling. The first assumption was that the spectra display the equilibrium between the monomeric form shown in the solid state by 17-19 and 21 and a dimeric form similiar to the structure shown by 20 in the solid state, reminiscent of the Schlenk equilibrium species. To test this assumption a DOSY spectrum was recorded.^[182,183] To our surprise the DOSY spectrum showed that both species move with nearly the same speed in solution, indicating that the size of both molecules and the atom numbers should be almost the same. With the help of the recorded DOSY spectrum it was also possible to calculate the size of the molecules in solution. The measurement showed that species a and b have a radius of approximately 4 Å. This radius fits the size of the monomeric form that could be observed in the solid state for 17-19 and 21. Because of these experiments and from our earlier experiences with magnesium imidosulfinates^[68,184] we think that the second species in solution might be [Mg{(N^tBu)2SR}2] or [(THF)Mg{(N^tBu)2SR}2]. All three compounds are about the same size and should yield very similar peaks in the NMR (see Figure 2-32).

Figure 2-32: Possible conversion of the magnesium diimidosulfinates in solution.

Additional evidence for this assumption provided a NOESY spectrum. This spectrum did not show defined cross peaks between both species but nevertheless cross peaks can be observed between both species and very broad signals next to the ^tBu peaks and the peaks for the methyl groups. This suggests that the exchange between both species a and b is mediated through an intermediate that is only short-lived and therefore only yields a very small and broad signal (see Figure 2-32). One possibility would be that this intermediate is a dimeric form similar to the structure of 20 in the solid state.

The experiments in this work showed that sulfur diimides react readily with different Grignard reagents. All products are obtained very pure and in high yields. By varying the steric demand of the substituent at the sulfur atom it is possible to get monomeric or dimeric molecules that can be distinguished by X-ray structure determination in the solid state and show interesting features in solution. The structural motifs of the monomer and the dimer are very different but in contrast to their lithium counterparts the monomeric and the dimeric magnesium diimidosulfinates do not show much variance among each other. The NMR spectra of 17-21 all show a multiple set of signals. It could be determined that the interconversion between the different species is temperature, time and concentration dependent and a reasonable explanation for the signal doubling supported by different NMR experiments was suggested.

Nevertheless, an unequivocal assignment to a single process in solution could not be made. Since the reactions of the Grignard reagents with the sulfur diimides proceeded very smoothly further studies on the reactions of Grignard reagents with sulfur triimides were made. Up to now reactions of organolithium reagents with sulfur triimides were only successful with small organic substituents because of the electron concentrations above and below of the SN3 plane.^[162] By the application of a HSAB-soft Grignard nucleophile penetration of the electronic shielding should be possible, because it matches the soft character of the sulfur atom.